We propose and experimentally demonstrate a generic framework, requiring neither active components nor complex designs, for undistorted asymmetric elastic-wave transmission in a thin plate using just a layer of lossless metasurface. The asymmetric transmission stems from the uneven diffraction of the +1st and −1st orders on opposite sides of the metasurface. Compared with previous loss-induced strategies, the present metasurface provides nearly total transmission on the transportation side, but total reflection on the opposite side, exhibiting a higher contrast ratio of the transmission. Moreover, we show that this strongly asymmetric behavior is robust with respect to the incidence angle, the frequency, and the effects of loss. The present work paves the way to compact rectification, high-resolution ultrasonography, and vibration and noise control in elastodynamics and acoustics.

A multistage damage detection method is introduced in this work that uses piezoelectric lead zirconate titanate (PZT) transducers to excite/sense the Lamb wave signals. A continuous wavelet transformation (CWT), based on the Gabor wavelet, is applied to accurately process the complicated wave signals caused by the damage. For a network of transducers, the damage can be detected in one detection cell based on the signals scattered by the damage, and then it can be quantitatively estimated by three detection stages using the outer tangent circle and least-squares methods. First, a single-stage damage detection method is carried out by exciting a transducer at the center of the detection cell to locate the damaged subcell. Then, the corner transducers are excited in the second and third stages of detection to improve the damage detection, especially the size estimation. The method does not require any baseline signal, and it only utilizes the same arrangement of transducers and the same data processing technique in all stages. The results from previous detection stages contribute to the improvement of damage detection in the subsequent stages. Both numerical simulation and experimental evaluation were used to verify that the method can accurately quantify the damage location and size. It was also found that the size of the detection cell plays a vital role in the accuracy of the results in this Lamb-wave-based multistage damage detection method.

This paper presents a crack detection method for fibre reinforced composite beams based on the continuous wavelet transform. Finite element method simulations and experiments were carried out for flawed cantilever composite beams subjected to impact load at their free ends. The Gabor and Morlet wavelets were chosen for wave signal processing owing to their good time-frequency characteristics. Both wavelets perform well in the crack detection of composite beams. The results show that the crack location can be determined successfully, and the crack depth can be estimated from the crack-reflection ratio. Moreover, the Gabor wavelet can locate the cracks more accurately by virtue of its higher time resolution.

A damage estimation method based on continuous wavelet transformation (CWT) of the normalized Lamb wave signals is proposed here. Lamb waves are actuated and sensed using piezoelectric (lead zirconate titanate, PZT) transducers arranged in the form of square detection cells on a plate-like structure. Excitation sequences based on pitch–catch and pulse–echo configurations are tested for the same arrangement of the transducers. The possibilities of the existence of damage for each actuator–senor pair are formed by using the normalized coefficient of CWT. The size of the possible damage region is directly controlled through envelopes defined by the coefficients of CWT, and no additional parameter is required to define its size. The aggregate damage image is constructed by the fusion of damage possibilities from all actuator–sensor pairs using damage indices based on conjunctive and compromised fusion schemes. The results indicate that the proposed method can estimate the location and severity of multiple damage with signals directly from the damaged plate, without the need of baseline signals from the undamaged plate, and the time-compensated signals provide better damage imaging than the raw signals. The most accurate and computationally inexpensive combination is the pulse–echo configuration with damage index based on conjunctive image fusion scheme. The method is computationally inexpensive and can be applied for multiple damage estimations in large structures to reduce the evaluation cost and inspection time during on-line structural health monitoring.

A novel resonator-based metasurface is devised to control reflected underwater waves. Each metasurface unit is constructed with an aluminum plate attached to a lead mass. By tailoring the thickness of the plate, full 2π phase shift of the reflected wave can be achieved. Examples of redirection, focusing, and directional carpet cloaking are demonstrated as applications. The target frequency can be as low as 500 Hz and it can operate in a broadband range. Slight modifications allow the design to perform excellently from tens to thousands of hertz. This metasurface provides a new design paradigm when there is a need to manipulate low frequency underwater waves, such as in submarines for stealth operation or detection-signal manipulation to provide deceptive information.

We investigate the effect of structural periodicity on elastic longitudinal and flexural wave propagation in parity-time ($\mathcal{PT}$) symmetric prototypical elastic metamaterial rods and beams. Through theoretical analysis and numerical demonstration, we reveal the link between the observed band structure of infinite periodic $\mathcal{PT}$ symmetric elastic metamaterials and the unique scattering properties of the corresponding finite periodic $\mathcal{PT}$ symmetric systems. In particular, we study the coherent perfect absorber and laser (CPAL). There is an observed periodic-like occurrence of the CPAL formation when varying the cell number in the finite periodic metamaterial that correlates to the band structure of the infinite periodic system. For this, we provide simple equations to estimate the periodicity and the minimum cell number. Using a piezoelectric-aided elastic metamaterial consisting of piezoelectric patches and active circuits, we propose a feasible configuration to realize tunable flexural wave CPALs in a periodic-structured $\mathcal{PT}$ symmetric metamaterial beam, and demonstrate that the CPAL can be induced in the Bragg's bandgap.